The annealing and recrystallizing temperature for steels is about 1,100 to 1,400 degrees Fahrenheit (590 to 760 degrees Celsius) and about 650 to 800 degrees F (345 to 425 degrees C) for aluminum. If metal is deformed plastically above the annealing temperature (for instance, forged or hot-rolled), then the hardening caused by the work is annealed immediately. Therefore, the mechanical properties of hot-worked material are very similar before and after the process, although the shape of the crystals changes.
Hot working requires less force, and it is possible to achieve more thickness reduction at each forming pass. During rolling, the metal thickness is reduced gradually, and the strip gets longer and longer, increasing its surface. After a while, the surface and the related heat loss become too large to continue the rolling at the elevated temperature. Cooling is usually the limiting factor for hot rolling. Therefore, hot-rolled steels typically are available only in thicknesses greater than 0.060 to 0.080 inch.
Carbon Steel
Pure iron is not used by the sheet metal industry. All commercially used steels have a small percentage of carbon, as well as different impurities absorbed by the iron during the steel-making process. Even a very small amount of impurities, such as sulphur or phosphorous, can influence (usually reduce) certain properties of the steel. Therefore, these impurities are controlled carefully.
Carbon also has a major influence on several properties. The so-called mild steels, with carbon content not exceeding about 0.20 percent, can be formed and welded easily, but their hardness and strength cannot be increased by heating and quenching (heat treatment).
Steels with higher carbon content (about 0.4 to 1.0 percent) have higher yield and strength and lower elongation. They cannot be welded with the usual processes, but they can be heat-treated. Many spring steels belong to this group.
The commonly used American Iron and Steel Institute (AISI) specifications, such as C1008, C1010, C1020, etc., refer to the carbon content. The carbon content of these steels does not exceed 0.08, 0.10, and 0.20 percent, respectively.
Because the carbon content and the mechanical properties are related to each other, designers sometimes specify the carbon content to achieve good formability or high strength. Controlling the chemical composition costs the mills more, so buyers must pay more for metal whose chemical composition has been specified.
Different metallic coatings have been developed to prevent rusting of carbon steels. Tin, chromium, zinc, aluminum, and other metals are used for this purpose. Metallic coatings can be applied, for example, by dipping the steel into liquid metal, or by the electrolytic process, vapor deposit, hot spray, lamination with fusion bond, or chemical bond. In all cases, preparing and cleaning the steel surface are essential.
